Photoionization of atoms with ultrashort XUV laser pulses at extreme intensities

The present thesis is concerned with the photoionization dynamics of atoms exposed to high intense and ultrashort XUV-laser pulses of free-electron lasers. In particular, the influence of coherent dynamics of the electrons as well as the electron-electron interaction are in focus of this survey and the resulting signatures as Rabi oscillations and characteristic spectra of photoelectrons are analysed. Furthermore, different levels of theory are applied for one-dimensional model atoms, providing a systematically manner to pinpoint the features induced by coherent and correlated electron dynamics. To this end, the exact solution of the time-dependent Schrödinger equation is determined numerically for a two electron system. To surmount the limitation of the latter to systems with a small number of electrons, the time-dependent reduced density matrix formalism is implemented on the Hartree-Fock level as well as one level above and thus accounts also for a correlated electron-electron interaction. Furthermore, for the ionization of resonantly driven atomic bound states an extended two level system is discussed, resulting in an analytic solution. As a prime example of coherent quantum dynamics Rabi oscillations are studied in a two electron atom for a resonant laser coupling between the ground state and an excited bound state. In addition a single-photon absorption of the model atom in the excited bound state gives rise to an ionization process. It was found that the ground state occupation as a function of time exhibits damped Rabi oscillations. The ionization, which induces the damping, scales linearly with the field intensity. For the ion yields induced by a finite laser pulse a quadratic intensity scaling is observed for pulse lengths below the Rabi period. Consequently, at a critical pulse area the onset of Rabi cycles induces a transition between a quadratic and linear intensity scaling in the ion yields. In the high intensity regime the comparison between the results of a simplified solvable model, including only two bound states coupled to the ionization continuum, and the exact solution of the time-dependent Schrödinger equation reveals that the further excited localized states and the direct two-photon ionization of the ground state carry a non negligible contribution to the total ionization yields. Besides, a shift of the Rabi frequency compared to its value for an isolated level is found. The analytic solution of the simplified model manifests that the shift is related to the ionization. Thus, the higher ion yields observed for the time-dependent Schrödinger equation affect the respective Rabi frequency, acquiring a high renormalization of the Rabi frequency of an isolated system. Investigations for an off-resonant excitation prove that the basic dependency of the Rabi amplitudes and of the Rabi frequencies on the detuning coincides with the one known for the isolated two level system. The detuning additionally affects the renormalization of the Rabi frequency. Depending on the sign of the detuning a Rabi frequency larger or smaller than in the isolated two level system is observed. However, the direct two-photon ionization of the ground state gains in importance already at small intensities compared to the resonant excitation and affects the intensity scaling of the ion yields. In particular, for a large detuning the transition from a quadratic to a linear intensity scaling is determined by a competition between the direct two-photon ionization and the Rabi-assisted ionization instead of the critical pulse area needed for one Rabi cycle. For a fixed detuning the respective intensity at which the transition occurs is the same for all pulse durations as long as the ion yields do not reach the saturation regime. In addition, also signatures of the coexistence of different Rabi processes are observed as beats in the time evolution of the occupation of the ground state. The influence of correlated electron dynamics on the ionization process has been studied for a laser excitation in the vicinity of the single-photon ionization threshold of a helium model. The comparison between the time-dependent Hartree-Fock theory and the exact numerical solution of the Schrödinger equation reveals that correlation effects gain in importance at a high radiation intensity with a photon energy close to the threshold and strongly affect the time evolution of the ground state population. Furthermore, the momentum distributions of the electrons emitted in a double ionization process have been analysed. The characteristic signatures of different double ionization processes allow to separate their respective parts in the total double ionization yield. In case of the coexistence of a sequential two-photon and a non-sequential three-photon double ionization the individual ion yields scale quadratically and cubically with intensity. However, even though the three-photon double ionization dominates at high intensities its signature is not observed in the total ion yields. Instead, a power law with an exponent between two and three is found. Studying the temporal evolution of the two-electron momentum distribution reveals a broad peak at short times which becomes narrower with ongoing time. This feature reflects the energy-time uncertainty and indicates a strong coherent regime at short times. A further analysis of the temporal evolution of the double ionization yields for both electrons leaving the atom in the same direction exhibits signatures of a recapture process. Finally, the time-dependent reduced density matrix formalism is applied for an atom interacting with an ultrashort laser pulse. To this end, the Bogolyubov-Born-Green-Kirkwood-Yvon hierarchy is truncated one level above the mean field level and therefore correlations between electrons are included. The reduced one particle matrix and the two particle correlations are represented by the finite element discrete variable representation. The numerically obtained time traces of the Hartree-Fock orbitals forming the ground state are analysed for two, four and six electron atoms excited by an intense ultrashort XUV laser pulse. In order to identify correlation induced processes in the ionization dynamics, results for correlated electrons are compared with the time-dependent Hartree-Fock theory. In this manner, signatures of the shake-off ionization process and the Auger decay are found in the time evolution of the occupation of the Hartree-Fock orbitals.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Kaiser, Benjamin

Contributors dc:contributor

• Axt, Vollrath Martin

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2063/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2063